DESCRIPTION: (from principal investigator's abstract) Perception of color depends on neural processes of eye and brain. The color and brightness of a single, isolated light are closely related to the light s physical characteristics, but the same light may be perceived to have a very different color and brightness appearance when viewed as part of a complex scene. A fundamental unsolved problem in vision is how the color and brightness appearance of a light depend on the context of other stimuli in view. While previous research has considered this problem, most studies have used very simple visual stimuli such as a test field superimposed upon a uniform background or within a uniform surrounding field. This work has revealed basic mechanisms affecting color perception but does not consider important neural processes that depend on visual stimulation that is more complex than a uniform background or surround. Recent studies show that color perception depends strongly on even sparsely represented chromaticities within a modestly complex, inhomogeneous visual stimulus. The proposed research will evaluate fundamental properties of color and brightness perception in complex scenes, by testing specific hypotheses concerning neural mechanism spatial, temporal and chromatic characteristics. The physiological locus of these neural mechanisms also will be investigated. This will be achieved by making psychophysical measurements of color perception with many different types of carefully designed inhomogeneous fields. The appearance of both increments and decrements will be studied in order to investigate the full range of color percepts that occur in natural scenes. Overall, this research will study neural processes that mediate color appearance under conditions closer to natural viewing than have been studied in most previous research. Reduced color vision in patients is typically assessed with tests of color matching or discrimination that use very simple fields. Neural mechanisms affected only by more complex fields, as occur in natural viewing, are not evaluated. A long-term goal of this research is to understand fully the neural processes mediating color and brightness perception of arbitrarily complex visual stimuli, and to develop tests to assess functioning of these neural mechanism.